XANTHAN PROPERTIES FOR SPECIFIC APPLICATIONS
3.3 STRATEGIES FOR CHANGING XANTHAN PROPERTIES THROUGH PHYSICOCHEMICAL PROCESSES
3.3.3 DEGRADATION OF XANTHAN CHAINS
The charge density of polysaccharide chains and the nature of counter ions play an important role in synergistic interaction and gel forma- tion (Annable et al., 1994). The interaction between the xanthan side chain backbone and glucomannan (Konjac mannan) in aqueous solu- tions occurs nearly instantaneously bellow the conformational transition temperature of xanthan, determining the associated formation which is thermodynamically favored, the driving force being the decrease of the number of xanthan/water contacts. The presence of electrolyte shifts the conformational change of xanthan to higher temperatures. Divalent cations present a more pronounced effect than monovalent ones because they are more effective for promoting the aggregation or ordering of xanthan chains. The polymer–polymer heterocontacts involve both ordered and disordered xanthan sequences. The gelation temperature decreases in the presence of electrolytes because the self-association of xanthan macromolecules is favored in a greater extent than xanthan/
Konjac mannan associations.
3.3.3.1 ULTRASOUND DEGRADATION
Comparatively with classical methods of polymer degradation (e.g., thermal, mechanical, and chemical degradation), many authors (Kulicke et al., 1996; Schittenhelm and Kulicke, 2000; Pfefferkorn et al., 2003;
Goodwin et al., 2011) agreed that ultrasonic degradation is an adequate method to control molecular weight and to produce fragments of defi- nite molecular size. The simplicity, inexpensively, environment friendly (especially because requires a short processing time), and lack of purifi- cation steps of degraded samples (does not need the addition of reagents) are few advantages of this method. During ultrasound degradation the polymer chain scission occurs close to gravity center of macromolecule (Kulicke et al., 1996) without major change in the chemical structure of xanthan. Milas et al. (1986) confirm the lack of changes in chemical structure of xanthan degraded by sonication (the substituent content in the xanthan side chain remains unchanged). They observed that by sonication a xanthan sample with low polydispersity index (around 1.4) was obtained; generally, the polydispersity ranges from 1.4 to 2.8. The ultrasound degradation rate depends on the frequency and intensity of ultrasound (Mark et al., 1998; Czechowska-Biskup et al., 2005; Koda et al., 2011; Price and Smith, 1993a), nature of the solvent (Price and Smith, 1991; Malhotra, 1982; Li and Feke, 2005a, 2005b), concentration of polymer solution and temperature (Price and Smith, 1993a, 1993b), and sonication time (e.g., before reaching a constant minimum value, the molecular weight decreases exponentially in time). A comprehensive study about the effect of temperature, salts, and pyruvate groups on ultra- sonic degradation of xanthan in aqueous solution (cp = 3.0 g/L) has been conducted by Li and Feke (2015a, 2015b). With regard to temperature, they observed a faster degradation rate or better degradation at 25°C than at 35°C. The increase of temperature favors both the disordered confor- mation of xanthan (that is the most flexible and the reactive structure with high degradation potential (Lambert and Rinaudo, 1985; Christensen and Smidsrød, 1991)) and the increase of the solvent vapor pressure. The vapor pressure slows down the movement of solvent molecules, the shock waves are diminished and so, the chains degradation is annihilated (Price and Smith, 1993a). The salt effect on xanthan degradation is complex and depends on specific behavior of salt in aqueous solution (e.g., salting- out or salting-in behavior according to the Hofmeister theory). Salts with
salting-out behavior inhibit the dissolution of polymers and so, decrease the efficiency of ultrasonic degradation, whereas the salts with salting-in behavior have opposite effect (Li and Feke, 2015a, 2015b). As mentioned above, the pyruvate groups decrease the stability of ordered conforma- tion of xanthan and consequently decrease the degradation efficiency of xanthan.
3.3.3.2 DEGRADATION INDUCED BY RADIATION
Irradiation represents a conventional method by which the polymer struc- ture can be modified by degradation, grafting, and cross-linking. Some- times, even a few cross-links or scission sites in the molecular structure can dramatically affect the strength or solubility of a polymer.
As a function of the radiation energy, ionizing and non-ionizing radia- tions exist. Ionizing radiation is made up of energetic subatomic particles (include alpha and beta particles and neutrons), ions, or atoms moving at high speeds (usually greater than 1% of the speed of light) and electro- magnetic waves on the high-energy end of the electromagnetic spectrum (gamma-ray, X-ray, and the high ultraviolet) while the low ultraviolet part of the electromagnetic spectrum as well as the part of the spectrum below UV including visible light, infrared, microwaves, and radio waves are considered non-ionizing radiations. Among ionizing radiation, gamma- ray enjoys great attention because can lead to polysaccharide degrada- tion (El-Mohdy, 2017; Charlesby, 1997; Li et al., 2011; Binh et al., 2016) without introducing chemical reagents or control temperature, environ- ment, and additives; the degradation of the polymer chain in aqueous solu- tion is produced through the free radicals resulted from water radiolysis (i.e., hydroxyl radical, hydrate electron, and hydrogen atom) (Al-Assaf et al., 1995).
The most important effect of polysaccharide degradation induced by gamma-ray is the scission of the C─O bond connecting the glycoside groups on the main chain (Şen et al., 2016) accompanied by the appear- ance of low molecular weight oligomers (Hayashi and Aoki, 1985; Ghali et al., 1979; Raffi et al., 1981). Such oligosaccharide with new functional characteristics (such as improved bioactivity and antioxidant and bioad- hesive activity) can be used to obtain biomedical products (Clough, 2001;
Rosiak et al., 1995) or in agriculture to enhance the crop yields and defense
system of the plants, to reduce the loss of chemical fertilizers, etc. (Luan et al., 2003; Binh et al., 2016).
Degradation by gamma irradiation was conducted on xanthan both in dry form (Şen et al., 2016) and in solution (Li et al., 2011; Binh et al., 2016). Şen et al. (2016) irradiated xanthan in solid state at different doses (2, 5, 5.0, 10, 20, 30, and 50 kGy) and dose rates (0.1, 3.3 and 7.0 kGy/h), and found that: (1) the chain scission is more effective at low dose rates and (2) irrespective of the dose rate and dose used the non-Newtonian and shear thinning behavior of xanthan solution (1% concentration) has not changed. Another effect of irradiation treatment in solid state is related to xanthan solubility; the radiation dose reduces the time required for dissolution and thus, increases the xanthan solubility (Binh et al., 2016).
Li et al. (2010) subjected the irradiation of 2% w/v xanthan solution (at different radiation doses and a dose rate of 0.5 kGy) and concluded that the degradation was caused by simultaneous action of gamma-ray on crystal- lization and amorphous area of xanthan. Therefore, the molecular weight (Mw) and the polydispersity (I) of xanthan decrease with gamma-ray dose from Mw = 5755.5 kDa and I = 3.3 at 0 kGy to Mw = 7.3 kDa and I = 2.4 at 120 kGy.
Comparing the molecular weight of xanthan obtained by irradiation (at the same dose and dose rate), it was observed that the xanthan degrada- tion is more efficient in solution than that in solid state (Binh et al., 2016;
Yan-Jie et al., 2010; Şen et al., 2016).
3.3.3.3 AUTOCLAVING EFFECT ON XANTHAN STRUCTURE Lagoueyte and Paquin (1998) have observed that the mechanical stress during high-pressure treatment can degraded xanthan chains; the degrada- tion was evidenced by continuous reduction of xanthan molecular weight as well as solution shear viscosity.
Later, Gurlez et al. (2012) confirmed the degradation of macromolec- ular chains at high pressures when the xanthan solution was submitted to different treatments (such as heating, high-pressure homogenization, auto- claving, and irradiation) and they reported a similar effect of autoclaving in the case of oat-glycan. The effects of these treatments are presented in Figure 3.4.
FIGURE 3.4 Schematic illustration of structural and conformational changes of xanthan in aqueous solution due to the effect of high pressure and autoclaving. Reprinted with permission from Gulrez, S. K. H., et al. Revisiting the Conformation of Xanthan and the Effect of Industri ally Relevant Treatments. Carbohydr. Polym. 2012, 90 (3), 1235–1243.
© 2012 Elsevier.
3.3.3.4 CHEMICAL DEGRADATION
Chemical degradation of xanthan can be induced by acid, alkaline, and enzymatic processes.
Christensen et al. (1991, 1993, 1996) studied the xanthan degradation by acid hydrolysis (e.g., pH = 1–4) at 80°C when the xanthan conformation was varied from fully ordered to partially disordered. Thus, depolymer- ization of double-stranded xanthan by partial acid hydrolysis can lead to xanthan fractions with conformational properties more or less analogs to undegraded xanthan (depending on ionic strength and degradation time), various chemical compositions of the side chains and reduced molecular
weights. In the side chain the terminal β-mannose hydrolysis occurs prefer- entially and leads to modified xanthan fractions from the intact “polypen- tamer” to “polytetramer”; the inner α-mannose acidic hydrolysis is slow while the glycosidic bond between glucuronic acid and α-mannose is resis- tant to hydrolysis. “Polytetramer” fractions obtained from xanthan hydro- lysis present low viscosity, while the lack of the two terminal units from the side chains (β-mannose and glucuronic acid) leads to a “polytrimer”
with higher viscosity (resulted by enzymatic hydrolysis). The acetyl group has a negative effect on the “polytetramer” viscosity and does not affect the
“polytrimer” xanthan viscosity (Hassler and Doherty, 1990).
Enzymes are substances that act as catalysts in living organisms, regu- lating the rate at which chemical reactions take place without their alter- ation. They are constituted by linear chains of amino acids that fold to produce a three-dimensional structure.
Even if xanthan is a polysaccharide highly stable to degradation induced by most of microorganisms, it can be degraded by cellulase (Rinaudo and Milas, 1980), as well as xanthanase (endo-1,4-β-D-glucanase) and xanthan lyase (4,5-transeliminase) found in microbes. Some xanthanases have been categorized as the cellulase family members. Taking into account that the biodegradation processes described in literature (Ahlgren, 1991;
Shatwell et al., 1990; Sutherland, 1982, 1987; Hashimoto et al., 1998) are associated with those of hydrolysis, xanthanase catalyzes the hydrolysis of the xanthan backbone, whereas xanthan lyase catalyzes the cleavages of the link between the terminal mannosyl and glucuronyl residues from the side chain of xanthan.
The degradation process produced by enzyme is followed by changes in molecular mass, viscosity and morphology. Thus, Rinaudo and Milas (1980) demonstrated that in salt-free solution the rate of enzymatic hydro- lysis of xanthan is reduced by the presence of aggregations or microgel structure; the xanthan chains in unordered conformation present a higher ability to degrade because small side chains protect the glycosidic bond against hydrolysis.
Another way to degrade the xanthan structure is based on chemical reactivity of acetyl group making possible the xanthan deacetylation by alkaline treatment.
The literature shows that, irrespective of deacetylation conditions (i.e., 0.025 M NaOH solution for 3 h (Covielo et al., 1986; Dentini, 1984), 0.01 M KOH solution for 10 h (Tako and Nakamura, 1984, 1989; Tako,
1991) or for 2.5 h (Khouryieh et al., 2007)), the process occurs without further modification of xanthan backbone. The deacetylation degree and the viscosity values at 10 s−1 of xanthan solutions in the two types of alka- line solutions were close (1.3%, 410 mPa⋅s in 0.01 M NaOH and 1.4%, 420 mPa⋅s in 0.01 M KOH) (Pinto et al., 2011).
3.3.4 CHEMICAL FUNCTIONALIZATION OF XANTHAN